This invention relates to an active piezoelectric spindle bearing preload adjustment mechanism and particularly a mechanism that is capable of generating electric power through rotation of the shaft in the a spindle to control the contracted deformation of a piezoelectric material for adjusting bearing preload alteration
Increasing demands for machining precision on machine tools and great expansion of machining applications have created a lot of challenge to the fabrication of high-speed spindle. One of the manufacturing issues pending to be resolved is the generation of thermo power efficiency of the spindle. For instance, the thermo source originated from a driving motor and preload friction of the rotating elements such as bearing increases rapidly when rotation speed accelerated. Furthermore, high-speed rotation of the spindle generates centrifugal force which makes bearing components such as inner and outer rings and steel balls squeezing against each other and produces thermal stress which in turn increases bearing preload. The increased bearing preload further makes the rotating bearing generating even more thermo energy. This vicious cycle thus makes bearing internal thermo energy increasing at accelerating rate and may make bearing preload become exceedingly high. If increasing of bearing preload is done without proper control of thermal stress, the bearing would be eventually burned out and destroyed. Bearing service life and durability will be suffered.
Hence how to reduce the increase of spindle bearing preload caused by thermal expansion is an important issue in the research and development of high-speed spindle technology. Many new technologies regard bearing preload adjustment have been announced and introduced over the years. In general, bearing preload method may be categorized in three types, i.e. fixed position preload, constant pressure preload and variable preload.
In the fixed position preload technique, a fixed dimension element such as spacer ring is disposed between the bearing and a stationary block. The spacer ring presses the bearing to provide the bearing a selected preload for increasing the rigidity and supporting capacity of the spindle. When the spindle rotates at a relatively low speed, this type of preload technique may provide the spindle a desirable rigidity. However when the spindle rotates at high speed, temperature will increase and may result in thermal expansion and preload overshoot and causes bearing failure.
In the constant pressure preload technique, the spindle is subjected to a constant axial pressure for providing the spindle a desirable preload. Using spring to render the preload to the spindle is a commonly used method for high-speed spindle at present. The spring can provide a constant preload. When bearing preload changes due to the factors such as rotation speed change or temperature increase, the spring may absorb the excess preload by its small displacement and almost does not increase preload value. However when using this method for preloading spindle bearing of machine for low speed and heavy duty machining work, the spring cannot provide the bearing sufficient load to increase the rigidity and supporting capacity of the spindle. Not enough rigidity will affect machining precision. Hence spring preload is only suitable for high-speed spindle.
Variable preload technique is to overcome the disadvantage of rigidity inadequacy of the spring preload mechanism. One of the variable preload techniques is using clutch principle by adapting the constant position and constant pressure mechanism on the spindle bearing. When the spindle rotates at low speed, the clutch is actuated to switch to the fixed position preload mechanism for providing the bearing a higher rigidity and supporting capacity. When the spindle rotates at high speed, switch to the constant pressure preload mechanism for providing the bearing a lower preload to prevent spindle rigidity overshoot. This technique is simpler and easier to implement. However it needs more space. Moreover, precision control is difficult. Hence it is not commercially available at present. Another variable preload technique is hydraulic preload mechanism which is widely used now. FIG. 1 illustrates its main features. There is a hydraulic cylinder assembly 2 engaging with the spindle 1 externally. The piston rod 21 can move reciprocally to control the axial displacement of the outer ring 111 of the bearing 11 for controlling the bearing preload. This technique needs an additional hydraulic source and other peripheral equipment. It costs higher and also needs a lot of space. The design of the spindle 1 has to include many more factors. Furthermore, preload level of the hydraulic preload mechanism is easily affected by pressure pulse. Once hydraulic source is not effective, the preload value will change significantly and may result in damaging the spindle 11.
FIGS. 2 and 3 show another known technology. It is an externally powered piezoelectric type preload control mechanism which has a piezoelectric material 3. When the piezoelectric material 3 subjects to an electric field, it will extend slightly along the electric field direction for controlling bearing preload. When the shaft 12 rotates at low speed, external power source provides a higher DC voltage for the piezoelectric material 3 to extend axially and push a preload adjustment block 4 for changing bearing 11 slide distance whereby providing sufficient bearing preload to maintain the rigidity of the shaft 12. When shaft 12 rotation speed and temperature increase, the external powered voltage is gradually decreased to reduce the extension of the piezoelectric material 3. In order to control bearing preload properly, bearing preload output value should be measured constantly by using a measuring device such as load cells made from strain gauge. Then a computer will be used to control the DC voltage and feedback to the piezoelectric material 3 for a close loop control process to adjust the bearing preload. This mechanism needs expensive external control devices and has a very complex structure. It becomes a roadblock to commercialization.
In view of aforesaid disadvantages, it is therefore an object of this invention to dispose a preload control assembly and an internal generator inside a spindle that are able to automatically adjust bearing preload according to spindle rotation speed without external control systems to measure and adjust preload output value whereby to increase reliability at a lower cost.
Another object of this invention is to enable the spindle to generate electric power during rotation so that the mechanism may function without external power supply and may reduce the costs of peripheral devices. It is also more environment friendly.
Still another object of this invention is to dispose the mechanism inside the spindle so that no additional space is needed.
In order to achieve aforesaid objects, this invention provides a preload adjustment assembly between the inner ring and outer ring of two bearings and has an internal generator disposed at a selected location in the spindle. The preload adjustment assembly includes a spacer ring set and a piezoelectric actuator located in the spacer ring set. When the spindle rotates, the internal generator produces electric power resulting from the spindle rotation and provides a voltage for controlling piezoelectric actuator extension length whereby to change slide distance of the inner and outer ring for controlling the preload value.